Plastic discs having a pit optical track structure are well known as compact discs. Double sided discs in which information is recorded on two sides of such a structure formed by binding two compact discs and apparatus for playing such discs is also well known, as in U.S. Pat. Nos. 4,903,140; 4,939,011; 5,097,465; 4,811,324. In forming such discs it is also known from U.S. Pat. Nos. 5,284,538 and 5,378,517 that it is advantageous to bring the two disc portions together at an angle in order to facilitate bonding.
The term compact disc as used in this application includes any disc shaped recorded medium having pits and lands forming an optical track structure on a data information recording planar surface. Thus it includes such discs where the information stored as pits and lands is digital information, analog information, or information of a graphic or holographic nature. Examples of the different types of information storage are as follows: Digital information may be encoded into the pits and lands as pits of quantized lengths interpretable as a stream of digital data, wherein the length of each pit or land segment may be, for example, related to the number of 1's or 0's in a data stream. An example of such a system is the compact disc ("CD") audio or video disc. Analog information may be encoded into the pits and lands as pit lengths that may vary over a continuum of values, wherein the length of each pit or land segment may be, for example, related to a voltage level in a desired output. An example of such a system is the laser disc video recording system. Graphics may be placed on the surface of a disc by employing the light reflection properties of pitted surfaces that are visible to the unaided eye. Similarly holographic information may be stored in a pit/land format, particularly where it is possible to vary the pit depth to create reflection interference patterns that are visible as a holographic image viewable by the unaided eye.
In-line manufacturing systems are utilized for mass producing copies of compact discs. These systems are capable of producing a compact disc every few seconds once a master disc has been produced. The process employs injection molding, vacuum metalizing, electroplating printing and if required, protective lacquer coating stages and robotic mechanisms for moving the discs from station to station.
An important requirement in the formation of certain double sided discs is the need to bond together two extremely thin discs to form the composite. Thin discs enable the encoding of data using reduced pit dimensions. Where a thin disc is produced it is necessary to provide additional thickness by bonding a plastic surface to the compact disc even where the plastic surface is not a compact disc in the sense that it does not separately encode data.
The composite must remain its planarity in order to be readable, and the bonding material must be applied uniformly and completely across the surfaces to be bonded. In the context of an in-line process it must be expected that the two surfaces may arrive at the bonding station at different temperatures, reflecting different lengths of time since heat generating processed. It is therefore necessary to compensate for these temperature differences so that a deformation of the bonded structure does not occur as the double sided disc achieves a uniform temperature and thereby to avoid a potato chip shape for the resulting composite disc. For purposes of this disclosure, a composite formed by bonding that has either one or two information bearing surfaces will be referred to as a double sided compact disc.